Mono-BHT heteroleptic magnesium complexes: synthesis, molecular structure and catalytic behavior in the ring-opening polymerization of cyclic esters

Abstract

Numerous heteroleptic 2,6-di-tert-butyl-4-methylphenolate (BHT) magnesium complexes have been synthesized by treatment of (BHT)MgBu(THF) with various alcohols. Molecular structures of the complexes have been determined by X-ray diffraction. The magnesium coordination number in [(BHT)Mg(μ-OBn)(THF)] (3) and [(BHT)Mg(μ-O-tert-BuCH)(THF)] (4) is equal to 4. Complexes formed from esters of glycolic and lactic acids, [(BHT)Mg(μ-OCHCOOEt)(THF)] (5) and [(BHT)Mg(μ-OCH(CH)COOCHCOOBu)(THF)] (6) contain chelate fragments with pentacoordinated magnesium. Compounds 3-6 contain THF molecules coordinated to magnesium atoms. Complex {(BHT)Mg[μ-O(CH)CON(CH)]} (7) does not demonstrate any tendency to form an adduct with THF. It has been experimentally determined that complexes 3 and 5 are highly active catalysts of lactide polymerization. The activity of 4 is rather low, and complex 7 demonstrates moderate productivity. According to DOSY NMR experiments, compounds 3 and 5 retain their dimeric structures even in THF. The free energies of model dimeric [(DBP)Mg(μ-OMe)(Sub)] and monomeric (DBP)Mg(OMe)(Sub) products on treatment of [(DBP)Mg(μ-OMe)(THF)] with a series of σ-electron donors (Sub) have been estimated by DFT calculations. These results demonstrate that the substitution of THF by Sub in a dimeric molecule is an energetically allowed process, whereas the dissociation of dimers is energetically unfavorable. DFT modeling of ε-CL and (dl)-lactide ROP catalyzed by dimeric and monomeric complexes showed that a cooperative effect of two magnesium atoms occurs within the ROP for binuclear catalytic species. A comparison of the reaction profiles for ROP catalyzed by binuclear and mononuclear species allowed us to conclude that the binuclear mechanism is favorable in early stages of ROP initiated by dimers 3 and 5.

Osteogenic Differentiation of Human Adipose Tissue-Derived MSCs by Non-Toxic Calcium Poly(ethylene phosphate)s

“…Benzyl alcohol (BnOH, 99%, Acros Organics, Geel, Belgium) and tert-butyl alcohol ( t BuOH, 99%, Merck, NJ, USA) were distilled over BaO and stored under argon. 2-Chloro-2-oxo-1,3,2-dioxaphospholane [45], 2-tert-butoxy-2-oxo-1,3,2-dioxaphospholane t BuOEP [23], and BHT-Mg catalyst [(BНТ)Mg(µ-OBn)(THF)] 2 [46] were prepared according to previously described methods (for details, see Section S1 in the Supporting Information).…”

Structural diversity of polynuclear Mg x O y cores in magnesium phenoxide complexes

“…Two bridging benzyloxide ligands connect the two Mg atoms, forming a nearly flat Mg 2 O 2 rhomboid core ( Fig. 2a), analogous to those observed in [(BHT)Mg(OBn)(THF)] 2 (Nifant'ev et al, 2017) Trinuclear complex (II) is located on a twofold crystallographic axis (Z 0 = 0.5) ( Fig. 3)…”

“…For example, the complexes (BHT) 2 Mg(THF) 2 (Calabrese et al, 1988; THF is tetrahydrofuran), n BuMg(BHT)(THF) 2 and [(-EtO)Mg-(BHT)(THF)] 2 (Nifant'ev et al, 2016), activated with alcohols, have demonstrated high efficiency in the ROP of lactide, "-caprolactone and !-pentadecalactone (Nifant'ev et al, 2016;Wilson et al, 2014Wilson et al, , 2015Fang et al, 2012;Chen et al, 2012). Using [(-RO)Mg(BHT)(THF)] 2 [R = Et or benzyl (Bn)] as examples, it has been demonstrated that activation with an alcohol is not necessary if both phenoxy and alkoxy groups are present within the same Mg complex (Nifant'ev et al, 2016(Nifant'ev et al, , 2017. Therefore, new heteroleptic (ArO)Mg(OAlkyl) complexes are of interest.…”

“…Oligomeric complexes usually demonstrate poorer catalytic performance than monomeric complexes [see, for example, Wang et al (2012) and Gao et al (2014)]. The dimeric complex [(BHT)Mg(-OBn)(THF)] 2 , which has been obtained recently by our group (Nifant'ev et al, 2017), has demonstrated significant catalytic activity in the ROP of cyclic esters, "-caprolactone and lactides. In order to determine the possibility of destroying its dimeric structure, we have performed the reaction between the complex [(BHT)Mg(-OBn)(THF)] 2 and the chelating -electrondonating solvent 1,2-dimethoxyethane (DME) in toluene (see Scheme, top).…”

“…Toluene was distilled from sodium/benzophenone ketyl. (Nifant'ev et al, 2017) and [(BHT) 2 Mg(THF) 2 ] (Calabrese et al, 1988) were synthesized according to the literature procedures.…”